It is known that oxocarbons typified by squaric acid have high acidity because its structure that hydrogen has been dissociated from an oxocarbon group is stabilized by resonance (Oxocarbons, p. 45 (Edited by Robert West), Academic, Press (1980), (ISBN: 0-12-744580-3)) (Journal of the American Chemical Society, 95, 8703 (1973)).
On the other hand, for example, a polymer compound having a sulfonic acid group is known as a polymer electrolyte useful in a polymer electrolyte type fuel cell etc. For example, there have been proposed, as polymer electrolytes useful in a polymer electrolyte type fuel cell etc., a fluorinated polymer such as Nafion (trademark of Du Pont Corporation), a polymer that a sulfonic acid group is introduced into polyetherketones (U.S. Pat. No. 5,433,082), a polymer that a sulfonic acid group is introduced into polyethersulfones (J. Membrane Science, 83, 211 (1993)), a polymer that a sulfonic acid group is introduced into polyimides (Japanese Patent Application 2003-277501), a polymer that a sulfonic acid group is introduced into polyphenylenes (U.S. Pat. No. 5,403,675), and a polymer that a sulfonic acid group is introduced into polyphosphazenes (Chemical Material, 3, 1120 (1991) 4).
Further, in a fuel cell repeating start and stop, it is known that a polymer electrolyte membrane is dilated with water generated in operation and shrinks due to drying in shutoff, causing a dimensional change.